1. Field of the Invention
The present invention relates to an image reading lens which is suited for the use at about same magnification (approximately ×0.6 to ×1.7), and relates to an imaging optical system and an image reader.
2. Description of Related Art
An image scanner is known, which is to take an image of a material such as a film, arranged at an effective distance, through an imaging lens in the imaging device such as a CCD (charge coupled device) and produce an electronic image therefrom. Meanwhile, digitization has being proceeded in the field of moving pictures. Such image readers have been developed as those to change the electronic image displayed on a display device such as an LCD (liquid-crystal display) into an optical image by an imaging lens and print it onto a moving-picture film and those conversely to take an image of a moving-picture film into an imaging device through the imaging lens and change it into an electronic image. Particularly, in the case such an image reader is used to capture a film image into the imaging device or so, there is a request to detect a microscopic scratch, a dust or the like as image information, and to digitally remove unwanted regions of images caused due to such a scratch, a dust or the like by image processing. It is a frequent practice to use near-infrared light in detecting a scratch, a dust or the like on a film. For this reason, an image reading lens preferably has a well optical characteristic in a wavelength range of from a visible to near-infrared region of light (e.g. approximately 900 nm), particularly a chromatic aberration well corrected both in magnification (i.e., lateral color) and on-axis (i.e., axis chromatic aberration). Meanwhile, it is preferable that aberrations, including distortion, are well corrected to provide the screen with high resolution in an area from a center to peripheral region thereof. Furthermore, where arranging a display device, such as an LCD, on the object side, it is frequently preferable to provide telecentricity to the object side. Meanwhile, where arranging an imaging device, such as a CCD, on the image side, it is frequently preferable to provide telecentricity on the image side. This is because the electronic device, such as an LCD and a CCD, in most cases has a dependence-upon-angle in the display or imaging characteristic thereof. In case a ray of light has an angle on the side where the electronic device is arranged, there is a possibility to cause so-called shading, i.e. lightness differing at between a center and a periphery of the image, thus deteriorating image quality.
Image reading lenses, corrected for aberrations in a range of from a visible to near-infrared region, include a lens described in JP-A-2002-148514 and JP-A-2002-287022. JP-A-2002-148514 describes an example that is wholly structured with six groups and fourteen lenses wherein the two lens groups, arranged on the object and image sides and adjacent to the aperture stop, are each structured with a cemented lens of three lenses. Meanwhile, JP-A-2002-287022 describes an example that is wholly structured with four groups and six lenses wherein the two lens groups, arranged on the object and image sides and adjacent to the aperture stop, are each structured with a cemented lens of two lenses.
The lens described in JP-A-2002-287022 is less in the number of lenses and advantageous in size reduction, but is insufficient in correcting aberrations where resolution is required higher. Meanwhile, the lens described in JP-A-2002-148514 is well corrected for aberrations by increasing the number of lenses. However, telecentricity is not provided at the object and image sides so that an electronic device cannot be suitably used. The lenses described in JP-A-2002-148514 and JP-A-2002-287022 are considered for the use only at a particular reference magnification wherein the imaging characteristic is not obtained well at a magnification other than the reference magnification. Accordingly, where applied to a moving-picture image reader for example, there is a need to inconveniently prepare a plurality of image reading lenses in order to cope with a plurality of film formats.